This invention relates generally to medical devices and, in particular, to a radially expandable stent.
Vascular stents are deployed at a narrowed site in a blood vessel of a patient for widening the vessel lumen and circumferentially supporting the vessel wall. Vascular stents desirably have a small cross-sectional diameter and/or profile for introducing the stent into the affected vessel lumen.
One type of a vascular stent is made with a piece of wire that is bent into a number of turns. Although suitable for its intended use, a problem with these bent wire stents is that stress points are formed at each wire bend or turn. As a result, the wire stent is structurally compromised at a For example, a wire stent is typically positioned in a blood vessel over an inflatable balloon. The balloon expands first at opposite ends, where the balloon is not in contact with the wire stent. As a result, the wire stent is longitudinally shortened between the inflated balloon ends. With continued inflation, the middle of the balloon expands, thereby unevenly expanding the wire bends of the longitudinally shortened wire stent.
Another type of a vascular stent is made with a wire mesh that is rolled into a generally tubular shape. A problem with this stent is that the overlapping wires forming the mesh increase the stent profile, thereby reducing the effective lumen of the blood vessel. The growth of endothelial tissue layers over the wire mesh further reduces the effective blood vessel lumen. Another problem with this approach is that ion migration also occurs at the wire-to-wire contact points.
Yet another type of a vascular stent is made with a flat metal sheet with a number of openings formed in rows therein. The flat metal sheet stent also includes rows of fingers or projections positioned on one edge of the stent along the axis thereof. When expanded, a row of the fingers or projections is positioned through a row of openings on the opposite edge of the stent for locking the expanded configuration of the stent. A problem with the use of the flat metal sheet stent is that the overlapping edges of the stent increase the stent profile. Again, the stent profile and endothelial growth reduce the effective blood vessel lumen. Another problem with the use of the flat metal sheet stent is that the fingers or projections along one edge of the stent make metal to metal contact with the opposite edge of the stent. As a result, the metal edges of the stent rub during movement caused by blood flow, pulsation, and muscle movement. Yet another problem with the use of the flat metal sheet stent is that the fingers or projections extend radially outward and into the vessel wall. As a result, the intimal layer of the vessel wall can be scraped, punctured, or otherwise injured. Injury and trauma to the intimal layer of the vessel wall result in hyperplasia and cell proliferation, which in turn effect stenosis or further narrowing of the vessel at the stent site.
Still yet another type of a vascular stent is made with a piece of metal cannula with a number of openings formed in the circumference thereof. A problem with the use of a metal cannula stent is that the stent is rigid and inflexible. As a result, the stent is difficult, if not impossible, to introduce through the tortuous vessels of the vascular system for deployment at a narrowed site. Furthermore, the stent is too rigid to conform with the curvature of a blood vessel when deployed at an occlusion site. Another problem with the use of a metal cannula stent is that the stent longitudinally shrinks during radial expansion. As a result, the position of the metal cannula stent shifts, and the stent supports a shorter portion of the blood vessel wall than required.
Previous attempts to overcome flexibility problems associated with cannula stent designs have included the addition of a flexible or articulation region between the relatively rigid segments. In comparison, these flexible regions or articulations provide little radial strength. There have been clinical concerns regarding the tendency of some cannula stent designs to plastically deform at the articulations during lateral bending rather than elastically returning to the original shape. Another concern is non-uniform radial expansion of the stent during balloon inflation. A commonly observed problem with such designs is that the flexible segments do not deform outwardly in the same manner and to the same degree as the segments of higher radial strength. As a result, the stent material of the interconnection regions extends or xe2x80x9changsxe2x80x9d into the lumen of the stent (as defined by the more rigid sections). Particularly in a vascular stent, local blood flow turbulence can occur at these points that can contribute to thrombus formation.
Still another phenomenon that is especially a problem in expandable cannula type stents is the tendency of thin bars or struts to twist during expansion. Even minor manufacturing defects can create weakened bending points that contribute to this problem. A design that increases longitudinal and radial strength and stability, has fewer articulations, and evenly distributes bending stresses is less prone to twisting and non-uniform expansion. Distribution of bending stresses is also an important factor in determining a stent""s susceptibility to fatigue. Articulations designed to provide flexibility between tubular non-flexible sections are typically subject to stresses during deformation that can lead to breakage. The likelihood of breakage can increase when the articulation points are welded rather than being part of the cannula wall.
For coronary applications, the ideal stent would be thin-walled, of unitary construction to eliminate welds, and have high radial strength with good endoluminal coverage to prevent restenosis. In addition, the stent would have a low profile on the balloon to reach small vessels, yet would have a good expansion ratio with low recoil following delivery to prevent migration or becoming undersized for the diameter of the lesion. An ideal coronary stent would be able to follow tortuous vessels during introduction while maintaining its shape without plastically deforming. Another desirable property is the ability of the stent to remain crimped upon the balloon so that slippage does not occur and, as a result, eliminates the need for endcaps or another means to hold the stent on the balloon. Although high radial strength is needed, the ideal coronary stent must be able to be elastically flexible over millions of bending cycles to accommodate changes in the vessel due to systole and diastole. The ideal stent should deploy uniformly at the target site without twisting, migrating, or taking on an accordion or scalloped appearance, should retain its original axial length during deployment, and should be visible under radiographic imaging as an aid in placement. While most available stents can adequately meet a limited number of these objectives, design compromises have restricted the utility and efficacy of these stents for certain clinical applications.
The foregoing problems are solved and a technical advance is achieved in an illustrative radially expandable stent that exhibits advantageously high, expanded radial stiffness for vessel wall support and an advantageously low, lateral bending stiffness for good trackability and introduction through tortuous vessels. The stent includes an elongated member with a passage extending longitudinally therein that is radially expandable with, for example, a balloon catheter. Alternatively, the elongated member can be self-expanding and can be comprised of, for example, a nickel-titanium alloy material, which advantageously has a supraelastic property. The elongated member includes a first longitudinal segment including a plurality of cells. Selected of the cells each includes a first and a second longitudinal strut for maintaining the longitudinal integrity of the stent before, during, and after expansion of the stent. In this configuration, the first longitudinal segment has an expanded radial stiffness greater than 1.6xc3x9710xe2x88x922 lbs (force) per millimeter (length). In another aspect, when the selected cells include a closed cell structure, the expanded radial stiffness need be just greater than 4.87xc3x9710xe2x88x923 lbs (force) per millimeter (length). In still another aspect, the expanded radial stiffness is greater then 3.47xc3x9710xe2x88x922 lbs (force) per millimeter (length).
The elongated member also has an interconnection segment connected to the first longitudinal segment and has a expanded radial stiffness less than that of the first longitudinal segment. The interconnection segment provides advantageously lateral flexibility to the stent. When the selected cells include longitudinal struts, the longitudinal segment and interconnection segment have a combined lateral bending stiffness less than 6.0xc3x9710xe2x88x926 in-lb (force) per degree per millimeter (length). When the selected cells are of a closed scissor-jack configuration, the combined lateral bending stiffness need only be less than 5.3xc3x9710xe2x88x925 in-lb (force) per degree per millimeter (length). In still another aspect, the elastic bending stiffness is less than 3.3xc3x9710xe2x88x926 in-lb (force) per degree per millimeter (length).
In one advantageous configuration, the interconnection segment includes a plurality of interconnected curvilinear struts that form an approximately serpentine pattern. In another advantageous configuration, the interconnection segment includes a plurality of interconnected linear struts that form a zig-zag or sawtooth pattern.
One or more connecting struts or members are utilized to interconnect the longitudinal and interconnection segments. The lateral flexibility of the interconnection segment minimizes, if not eliminates, the stress at the connecting strut or member and provides the stent with a high degree of expanded radial stiffness and significant lateral flexibility which can be used for long periods of time in a pulsatile environment without causing fatigue and fracture of the stent. The length of the stent can be selected as desired by including a plurality of the longitudinal segments of which adjacent ones are interconnected by an interconnection segment.
To enhance the radiographic visibility of the stent, at least one end of the stent includes a radiopaque marker of, for example, gold. To further enhance the radiopaque visibility of the stent, a plurality of radiopaque markers are positioned at an end of the stent to indicate the orientation of the stent. This plurality advantageously provides the physician with the spacial orientation of the stent when being introduced through the vascular system.
In another aspect of the radially expandable stent, the longitudinal and interconnection segments are interconnected and structured such that the longitudinal segment has a higher expanded radial stiffness. In this case, the combined lateral bending stiffness is less than 3.33xc3x9710xe2x88x926 in-lb (force) per degree per millimeter (length). The expanded radial stiffness of the longitudinal segment can then be greater than 3.47xc3x9710xe2x88x922 lbs (force) per millimeter (length).
In another embodiment of the radially expandable stent, the first longitudinal segment includes a plurality of interconnected cells. Selected of these cells each includes a first and a second longitudinal strut that are interconnected by at least one pair of circumferentially adjustable members. The circumferentially adjustable members advantageously permit the circumferential expansion of the longitudinal segment with minimal change in the axial length of the longitudinal struts when the longitudinal segment is radially expanded. As a result, the longitudinal struts remain substantially parallel with the longitudinal axis of the stent. In one aspect, adjacent cells of the selected cells share a common first and second longitudinal strut with respectively laterally adjacent cells. In another aspect, the circumferentially adjustable members are xe2x80x9cUxe2x80x9d- or xe2x80x9cVxe2x80x9d-shaped to provide a scissor-jack configuration, which provides the stent with its high expanded radial stiffness.
The foregoing problems are also solved and a technical advance is achieved in an illustrative radially expandable stent having a longitudinal segment including, for example, a plurality of laterally interconnected closed cells that are formed from, or into, a tubular structure or cannula that has an advantageously high expanded radial stiffness and that changes length minimally, if at all, when expanded radially. The stent also has an interconnection segment that is connected to the longitudinal segment and provides the stent with advantageous lateral flexibility and a low elastic bending stiffness. Each cell has first and second parallel longitudinal bars or struts that are interconnected at each end by a circumferentially adjustable member. In an illustrative example, the opposing circumferentially adjustable members are inclined toward the center of the cell""s aperture. A series of the basic cells are laterally interconnected to form a tubular structure. When the tubular structure is radially expanded, such as by an inflatable balloon, the longitudinal struts remain substantially longitudinal to each other and circumferentially aligned while the circumferentially adjustable members open or unfold as their attachment points move apart, resulting in an increase in cell width. The action of the circumferentially adjustable members, much like that of a scissor jack, accounts for the increase in stent diameter. Because the longitudinal struts retain their alignment, the longitudinal segment maintains a stable axial length during expansion. The stent can also be self-expanding. One such self-expanding stent can comprise a nickel-titanium alloy material having, for example, a supraelastic property.
Radiopaque markers can be advantageously positioned at one or both ends of the pattern to aid the physician in positioning the stent under fluoroscopic imaging. In the illustrative embodiment of the invention, gold markers are placed in the apertures at the ends of each longitudinal strut; however, not all apertures need be filled.
In the preferred illustrative embodiment of the invention, the circumferentially adjacent closed cells are interconnected such that they share the longitudinal struts of adjacent segments. In an alternative embodiment each closed cell contains longitudinal struts that are not shared with adjacent segments, but rather are connected to the adjacent longitudinal strut by at least one short strut.
Within the illustrative example of the invention are included interconnection segments positioned between adjacent longitudinal segments. The interconnection segments advantageously permit lateral flexibility in what otherwise would be a substantially rigid stent and is advantageous for use in a site subject to great flexural forces such as the coronary arteries. In the illustrative example, the interconnection segment is comprised of a continuous series of serpentine bends that connect to the adjacent longitudinal segment via at least one short interconnection strut. In addition to permitting the stent to flex and thus be placed in a more tortuous-shaped lumen, a primary requirement of the bends of interconnection segment is that they do not interfere with the expansion of the longitudinal segments. The number, shape, thickness, and point of attachment of these serpentine bends can be varied depending on the qualities desired in the stent. The stent in the illustrative example for peripheral use has three points of attachment at between a longitudinal segment and adjacent interconnection segment such that every third serpentine bend is attached to a longitudinal segment, one third of the bends are attached to the opposite longitudinal segment, and the remaining third are unattached on both sides. In another illustrated embodiment for coronary use, there is a single point of attachment between a longitudinal segment and an interconnection segment that is 180xc2x0 opposed to the single attachment point on the same interconnection segment. This pair of attachment points are rotated in subsequent interconnection segments to increase stent flexibility.
In another embodiment of the stent, the interconnection segment is broadly attached to the longitudinal segment rather than by a series of struts, giving the appearance that the adjacent segments have been joined by a series of xe2x80x9cSxe2x80x9d- or xe2x80x9cZxe2x80x9d-shaped struts. The configuration or pattern of curvilinear struts that comprise the interconnection region in the illustrative example is repeated throughout such that the longitudinal struts of each internal closed cell is narrowly or broadly connected to a bend of the interconnection segment at one end with the other end of the longitudinal strut free of attachment.
The interconnection segment configuration found in the illustrative example can be mirrored in the adjacent interconnection region such that every other internal longitudinal strut is connected at both ends, while the alternating longitudinal struts are unattached on both ends.
The serpentine bends of the interconnection segment can be varied to permit different expansion and flexural properties. For example, the bending regions or fillets can be xe2x80x9ckeyholexe2x80x9d shaped like the bends of the circumferentially adjustable struts wherein the struts flair outward around the apex of the bend. This modification reduces bending stress and allows for slightly more expansion capability. The struts between the bends can be parallel to each other and in alignment with the longitudinal axis of the stent, or they can be inclined with respect to the stent""s longitudinal axis, either alternately as in the illustrated preferred embodiment, or identically inclined. Alternating inclination of these struts can partially or completely offset any shortening that takes place as the stent is expanded to the nominal diameter. While there is no net change in the length of the longitudinal segments, the unfolding of the curvilinear struts of the configuration of the illustrative example, causes the interconnection segment to shorten, thereby affecting the overall length of the stent. This change in length can be partially offset when the opposing curvilinear struts are angled toward each other in a manner such that as the stent is expanded initially, the opening of the bend forces the stent to lengthen slightly. As the stent continues to expand, the angle of the bends are opened further, which serves to draw the longitudinal segments back together and shorten the stent. The xe2x80x9cdelayxe2x80x9d that angling the curvilinear struts affords before the stent begins to longitudinally contract, reduces the total contraction that would otherwise occur if the struts were longitudinally aligned. By choosing the proper curvilinear strut design, it is possible to produce a stent with virtually no net change of length at the nominal or final diameter.
A further advantage of angling the struts of the interconnection segment is that the stent can become more flexible in the unexpanded state. This provides improved trackability while the stent is mounted on the balloon catheter and maneuvered through the vessel to the target site. Another concern is that struts aligned with the longitudinal axis of the catheter would be more likely to flip out of plane during bending of the balloon catheter and pose a risk of damage to the vessel.
The pattern of curvilinear struts of the interconnection segment can be oriented such that each broadly attaches to the longitudinal segment in the same manner, i.e., there is not a general serpentine waveform pattern as in the illustrative example. One advantage of this type of design is that the curvilinear struts do not deform as the stent is expanded, and, as a result, no change in the length of the stent occurs. As in the case of the pattern found in the illustrative example, the direction or orientation of the struts can be reversed in an adjacent interconnection segment.
A balloon catheter is used to radially expand the stent to engage the vessel wall surface and to maintain the vessel lumen in an open condition. The expanded stent advantageously has a minimal thickness for endothelial tissue to form thereover. As a result, the vessel lumen is advantageously maintained with the largest diameter possible.